Nope. Correction: In many states they are, but in limited amounts. In Texas, the answer is Nope. It's their state law.
Under Citizens United they can spend all they want themselves and with the same protections as any individual, but they have to disclose that they are the ones behind the spending and can't hand it over to a candidate.
That's caused some confusion.
http://www.supremecourt.gov/opinions/09pdf/08-205.pdf
(PDF Alert)
It does sound fair.
I'm against it in principle, because I believe political speech rights should be reserved for natural individuals and not legally created fictitious ones, be they corporations OR unions - both of which were addressed in the decision.
But fair warning about getting me on that soapbox. I had a couple hundred incredulous posts or so in the Citizens United threads at the time the decision came out.
The other side of the coin is your Right as a Person to have relevant information available to you, not conditionally, not censored. The focus should be on verifiable information being available, not obstruction by the powers that be because it conflicts with their interest.